Clinton Foundation spokesman Craig Minassian said the foundation "has been subjected to politically motivated allegations, and time after time these allegations have been proven false".

"None of this has made us waver in our mission to help people."

According to its website, the Clinton Foundation builds "partnerships between businesses, NGOs, governments, and individuals everywhere to work faster, leaner, and better". It was set up in 1997 by then-President Bill Clinton.

Nick Merrill, a spokesman for Ms Clinton, said the probe's goal was to "distract from the indictments, guilty pleas, and accusations of treason from [US President Donald] Trump's own people at the expense of our justice system's integrity. It's disgraceful, and should be concerning to all Americans."

Mr Trump has repeatedly called for investigations into his presidential rival. The investigation into the Clinton Foundation has nothing to do with the probe into Ms Clinton's use of a private email server, CBS News reports.

A common refrain heard at Mr Trump's campaign rallies was "lock her up", in reference to Ms Clinton.